MECA0525 : Vehicle dynamics


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MECA0525 : Vehicle dynamics
Pierre Duysinx
Research Center in Sustainable Automotive Technologies of University of Liege Academic Year 2021-2022
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Lesson 2: Suspension effects on cornering
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Bibliography
◼ T. Gillespie. « Fundamentals of vehicle Dynamics », 1992, Society of Automotive Engineers (SAE)
◼ J. Dixon. « Tires, Suspension, and Handling » 2nd edition. 1996, Society of Automotive Engineers (SAE)
◼ H. Heisler. Advanced Vehicle Technology. 2nd edition. Butterwoth Heinemann. 2002.
◼ W. Milliken & D. Milliken. « Race Car Vehicle Dynamics », 1995, Society of Automotive Engineers (SAE)
◼ J. Reimpel, H. Stoll, J. Betzler. The Automotive Chassis - Engineering Principles. Butterworth Heinemann. Oxford. 2001.
◼ J.Y. Wong. « Theory of Ground Vehicles ». John Wiley & sons. 1993 (2nd edition) 2001 (3rd edition).
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Layout
◼ Introduction ◼ Suspension characteristics
◼ Roll stiffness ◼ Roll center
◼ Definition ◼ Determination procedure
◼ Lateral load transfer
• Load transfer in the axle • Roll moment distribution • Roll angle
• Understeer gradient due to lateral load transfer
• Tire sensitivity to load transfer • Understeer gradient due to load transfer
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Layout
◼ Understeer gradient due to camber
◼ Camber thrust ◼ Camber change ◼ Understeer gradient due to camber change
◼ Roll steer
◼ Roll steer ◼ Understeer gradient due to roll steer ◼ Rear axle roll steer
◼ Lateral force compliance
◼ Lateral force compliance ◼ Understeer gradient due to lateral force compliance
◼ Aligning torque
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INTRODUCTION

Suspension effects on cornering
◼ So far our first theory has shown that the behaviour is dependent on the balance between the load/cornering coefficient W/Ca (called cornering compliance [deg/g]) of the front and rear axles

◼ Weight and cornering coefficients can be affected by suspension mechanics due to

◼ Lateral load transfer

◼ Change in camber angles

◼ Roll steer

◼ Lateral force compliance steer

◼ Aligning torque

◼ Effect of tractive force on cornering

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Roll Moment distribution
◼ For virtually all pneumatic tires, the cornering forces are nonlinearly dependent on the vertical load (load sensitivity) Straight line: Fy= 760 lb Cornering: Fy=680 lb
Gillespie Fig. 6.11 8

Roll Moment distribution
◼ Lateral load transfer occurs in cornering because of the elevation of the CG with respect to the ground.
◼ Because of the load sensitivity of tyre cornering force, the lateral force developed by the axle is affected in cornering.
◼ Thus to sustain the lateral forces, the slip angle has to be increased.
◼ For front tyres, greater slip angles lead to an understeer effect ◼ For rear tyres, greater slip angles lead to an oversteer
behaviour
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Roll Moment distribution
◼ The load transfer is in action on both front and rear axles. The relative importance of the load transfers will depend on the balance of the roll moments distributed over the front and the rear axles.
◼ More roll on front axles contributes to understeer ◼ More roll on rear axles contributes to oversteer
◼ Auxiliary roll stiffeners (stabilizer bars) alter the handling performance through the redistribution of the relative importance of the roll between the axles
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MECA0525 : Vehicle dynamics